The primary purpose of the invention is to increase the carrier frequency of a low noise reference frequency source while minimizing the loss in carrier-to-noise ratio. A principal application is for radar exciters, which demand the best possible noise performance to minimize the spreading of main lobe clutter into otherwise clutter free frequency regions. To achieve this performance, a typical exciter derives all operating frequencies from one or more low noise crystal reference oscillator operating in the 100 MHz region. Because the radar operates at microwave frequencies, however, this frequency must be increased.
Current techniques generally employ frequency multiplication of a reference oscillator to achieve the desired operating frequency. This multiplication is variously based on the use of such devices as varactors, step recovery diodes, transistors, and harmonic phase-locked loops. The principal disadvantage of these approaches is that they increase the phase noise by 20 log(N). That increase is 10 log(N) more than the multiple-oscillator technique of the present invention.
Cryogenically cooled resonators (sapphire ring resonators, dielectric resonators, superconducting thick film resonators, etc.) can be used to provide low noise performance directly at the desired output frequency but incur the complexity of cryogenic cooling of a relatively large device. Additionally, the technology of these devices is largely in early stages of maturity and generally regarded as higher risk. Performance under vibration is also a concern and generally has not been characterized.